Cyclones are used in the main for separating a gas/vapour phase from a liquid phase. In industry today, use is in general made of cyclones which are specially manufactured with regard to the operating conditions in which the cyclone is principally to work. Furthermore, cyclones known today are in most cases constructed in such a manner that they cannot be adapted to different operating conditions, at least not without a considerable effort, as a result of which they often, in the event of a change in the production process, come to be used in operating conditions which are not optimum.
U.S. Pat. No. 2,377,721, U.S. Pat. No. 3,516,551 and U.S. Pat. No. 4,225,325, for example, show that this last-mentioned problem has been known for a long time and that there is a number of proposals for solutions of said problem. Common to these known solutions is that the flow variations are compensated in close proximity of the actual inlet into the cyclone by, in the case of a small flow, reducing the flowthrough area by a throttling member in the inlet arrangement of the cyclone so that the inlet rate is kept almost constant in order to obtain an optimum separation. In the case of liquids with a tendency to foam such a solution proves to have undesirable disadvantages, since the throttling gives rise to a change in the flow pattern and causes a pressure drop which in turn, because of the vapour formation etc., leads to an uneven flow pattern in connection with the outflow inside the cyclone and is thus a cause of foam formation. Foam formation is very disadvantageous since it disturbs the flow conditions inside the cyclone and thus counteracts optimum operating conditions.
In U.S. Pat. No. 5,879,422 and U.S. Pat. No. 5,669,948 are shown an example for establishing correct flow conditions to a cyclone, where the inlet pipe is equipped with an interchangeable insertion pipe and thus adaptable to different operating conditions. The operator could thus be equipped with a set of different insertion pipes. However, replacement of the insertion pipes in the inlet is an operation which calls for interrupted operation of the cyclone. In these cyclones is each interchangeable insertion pipe optimised for establishing a flow velocity above 30 m/s and preferably at or above 40 m/s. If the flow velocity occasionally drops below a critical flow velocity, and a decreased gas separation is experienced, could either heating of the gas laden liquid stream or alternatively other liquids be added to the gas laden liquid stream in order to maintain the critical flow velocity. Often is steam used as this heating media or added other liquid. This would result in an increase in steam consumption and increase of costs. It is also contra productive to add steam into a process having the objective to separate steam. Said steam separated is most often to be used for heating chips before being fed to the digester. Any decrease in steam production from the cyclone calls for usage of expensive fresh steam that could be used for other purposes. Decreased gas separation is often experienced in pulp mills cooking wood chips from pine or spruce, i.e. softwood, where the spent liquor has a high content of foam producing extractives.